1. Field of the Invention
The present invention relates to a tunable external cavity laser diode and, more specifically, to a tunable external cavity laser diode using a variable optical deflector in which a refractive index varies according to an electrical signal.
2. Discussion of Related Art
In general, an external cavity is used to convert light of a single wavelength emitted from a light source such as a laser diode having a predetermined bandwidth to light of a specific wavelength. A Littman-Metcalf configuration and a Littrow configuration are typical external cavity configurations, and the method of converting into the light of a specific wavelength is applied to dye laser technology or optical characteristic measuring technology widely used in researches of spectroscopy.
FIG. 1 is a structure diagram illustrating a conventional Littman-Metcalf external cavity.
The Littman-Metcalf external cavity is composed of a lens 11 parallelizing a beam generated from a laser diode 10 having a wide wavelength range, a diffraction grating 12 for diffracting the parallel beam, and a reflective mirror 13 for reflecting the diffracted beam.
When the beam is generated from the laser diode 10, the beam is gathered in parallel through the lens 11, and the parallel beam is diffracted toward the reflective mirror 13 by the diffraction grating 12. An angle of the reflective mirror 13 facing the diffraction grating 12 is adjusted by an apparatus (not shown), so that only the light of a specific wavelength, which is incident perpendicular to the reflective mirror 13, among the incident light may be reflected to the diffracted grating 12 by the reflective mirror 13. The beam returned to the diffraction grating 12 is diffracted again by the diffraction grating 12 and goes back to the laser diode 10 through the lens 11.
While the beam having a predetermined wavelength 15a is incident perpendicular to the reflective mirror 13 to be reflected to the diffraction grating 12, When the reflective mirror 13 is rotated to be located at a reflective mirror 13′, the beam having another wavelength 15b is incident perpendicular to the reflective mirror 13′ to be reflected to the diffraction grating 12. Therefore, the wavelength of the beam returning to the laser diode 10 varies according to an arrangement angle of the reflective mirror 13. With this principle, the wavelength varies according to an angle of the reflective mirror 13.
As illustrated above, the Littman-Metcalf external cavity laser diode varies a wavelength by adjusting the angle of the reflective mirror, while the Littrow external cavity laser diode varies a wavelength by adjusting an angle of the diffraction grating.
FIG. 2 is a structure diagram illustrating a conventional Littrow external cavity.
The Littrow external cavity has a similar configuration to the Littman-Metcalf external cavity, but changes a wavelength not by adjusting an angle of a reflective mirror but by adjusting an angle of a diffraction grating 22.
When a beam is generated from a laser diode 20, the beam is gathered in parallel through a lens 21, and a beam having a specific wavelength among the beams gathered in parallel is diffracted according to an angle of the diffraction angle 22 and goes back to the laser diode 20 through the lens 21. In other words, since the wavelength of the beam returning to the laser diode varies according to an arrangement angle of the diffraction grating 22, the wavelength may be varied.
As described above, in order to select the beam having a specific wavelength, the conventional Littman-Metcalf or Littrow external cavity tunable laser rotates the reflective mirror or the diffraction grating mechanically to adjust an angle. Therefore, since the reflective mirror or the diffraction grating should be finely rotated in a mechanical sense, a high-precision rotating apparatus is required to select the specific wavelength. This may result in degraded laser stability, a larger size, a slow variable speed and the increased manufacturing cost.
While the afore-mentioned conventional technologies have advantages in terms of the arrangement and performance, they also have problems. In other words, the conventional technologies require a mechanical movement, have a narrow wavelength tunable range, and are difficult to reduce the module size. Therefore, there is a need of a new method of fabricating a light source, which may be applied to spectroscopy that requires a tunable wavelength and a WDM optical communication system that requires a wide range of the tunable wavelength, has no movement of a structure, facilitates miniaturization, and has a fast variable speed.